My invention relates to the preparation of hollow microspheres. In particular, my invention involves an improvement in the manufacture of hollow inorganic microspheres by spray drying from aqueous media, said improvement comprising the addition of a water-miscible organic liquid with a high boiling point to the solution of alkali metal silicate and polysalt to be spray dried. The resulting product has fewer imperfections such as holes and/or cracks, so that porosity is reduced.
Hollow microspheres can be prepared by spray drying solutions that contain nearly any film-forming substance. Spray drying is particularly suited to the preparation of hollow microspheres from solids dispersed in aqueous media. U.S. Pat. Nos. 3,796,777; 3,794,503 and 3,888,957 disclose hollow spheres prepared by spray drying alkali metal silicate solutions that have been combined with "polysalt" solutions, and then carefully drying the intermediate hollow spheres. The process by which these products are made must be tightly controlled to control the holes, cracks and other surface imperfections that contribute to porosity which is undesirable in these products.
It is an object of this invention to prepare hollow spheres with less porosity while allowing more latitude in processing conditions.